Polystyrene-based ion exchange materials such as sulfonated polystyrene have been widely used in electrodialysis for water purification, desalination, chemical purification and treatment of industrial wastes. However, due to the lack of chemical and oxidative stabilities these materials tend to lose mechanical strength in use, and operation temperature has to be limited to less than 40.degree. C. Operation temperatures of 70.degree. C. or higher are desirable to reduce operation cost. Increasing the operating temperature eliminates refrigeration necessary for cooling, and results in at least a 30% decrease in power consumption. This requires membranes that retain good transport performance at high temperature, have better mechanical properties and chemical stability. Improved membrane materials also reduce the need for membrane replacement and extend their service time.
Generally, it is simpler to use a commercial resin and convert it into ion exchange material by introduction of functional groups. The base polymer must be reactive to enable easy introduction of ionic functional groups, and soluble to allow membrane casting from solutions. It also must be mechanically tough, and oxidatively stable in use. Hydrolytic stability is also important for membranes to be used in electrodialytic water splitting processes, since they are usually exposed to high concentrations of acid or base for long times.
Most commercially available polymeric materials fail to fulfill these requirements. Styrene-based polymers have low oxidation resistance. Polyamides, polyimides and polyesters are prone to hydrolysis. Most fluorine-containing polymers are insoluble or difficult to functionalize. The same is true of polyethylene-like polymers. Although sulfonated polysulfone has excellent mechanical strength and oxidative stability, it lacks resistance to hydrolysis by strong base. Sulfonated polyphenyleneether has outstanding transport properties, but possesses even worse oxidative stability than styrene-based material. Thus, the commercially available resins all possess serious shortcomings.
Arylenediisopropylidene copolymers are known. U.S. Pat. No. 3,770,661, 1973 and A. Fritz and R. W. Rees, Journal of Polymer Science Part A-1, Vol. 10, pp. 2365-2378 (1972).
Electrodialysis cells having ion exchange membranes are known. Uses of such membranes in electrodialysis cells have been reviewed in Synthetic Polymeric Membranes by R. E. Kesting, McGraw-Hill Book Company, New York (1985).